Heavy or settlement-sensitive facilities that are located in areas containing soft or weak soils are often supported on deep foundations, consisting of driven piles or drilled concrete columns. The deep foundations are designed to transfer the structure loads through the soft soils to more competent soil strata.
In recent years, aggregate columns have been increasingly used to support structures located in areas containing soft soils. The columns are designed to reinforce and strengthen the soft layer and minimize resulting settlements. The columns are constructed using a variety of methods including the drilling and tamping method described in U.S. Pat. Nos. 5,249,892 and 6,354,766; the driven mandrel method described in U.S. Pat. No. 6,425,713; the tamper head driven mandrel method described in U.S. Pat. No. 7,226,246; and the driven tapered mandrel method described in U.S. Pat. No. 7,326,004; the disclosures of which are incorporated by reference in their entirety.
The short aggregate column method (U.S. Pat. Nos. 5,249,892 and 6,354,766), which includes drilling or excavating a cavity, is an effective foundation solution when installed in cohesive soils where the sidewall stability of the hole is easily maintained. The method generally consists of: a) drilling a generally cylindrical cavity or hole in the foundation soil (typically around 30 inches); b) compacting the soil at the bottom of the cavity; c) installing a relatively thin lift of aggregate into the cavity (typically around 12-18 inches); d) tamping the aggregate lift with a specially designed beveled tamper head; and e) repeating the process to form an aggregate column generally extending to the ground surface. Fundamental to the process is the application of sufficient energy to the beveled tamper head such that the process builds up lateral stresses within the matrix soil up along the sides of the cavity during the sequential tamping. This lateral stress build up is important because it decreases the compressibility of the matrix soils and allows applied loads to be efficiently transferred to the matrix soils during column loading.
The tamper head driven mandrel method (U.S. Pat. No. 7,226,246) is a displacement form of the short aggregate column method. This method generally consists of driving a hollow pipe (mandrel) into the ground without the need for drilling. The pipe is fitted with a tamper head at the bottom which has a greater diameter than the pipe and which has a flat bottom and beveled sides. The mandrel is driven to the design bottom of column elevation, filled with aggregate and then lifted, allowing the aggregate to flow out of the pipe and into the cavity created by withdrawing the mandrel. The tamper head is then driven back down into the aggregate to compact the aggregate. The flat bottom shape of the tamper head compacts the aggregate; the beveled sides force the aggregate into the sidewalls of the hole thereby increasing the lateral stresses in the surrounding ground.
The driven tapered mandrel method (U.S. Pat. No. 7,326,004) is another means of creating an aggregate column with a displacement mandrel. In this case, the shape of the mandrel is a truncated cone, larger at the top than at the bottom, with a taper angle of about 1 to about 5 degrees from vertical. The mandrel is driven into the ground, causing the matrix soil to displace downwardly and laterally during driving. After reaching the design bottom of the column elevation, the mandrel is withdrawn, leaving a cone shaped cavity in the ground. The conical shape of the mandrel allows for temporarily stabilizing of the sidewalls of the hole such that aggregate may be introduced into the cavity from the ground surface. After placing a lift of aggregate, the mandrel is re-driven downward into the aggregate to compact the aggregate and force it sideways into the sidewalls of the hole. Sometimes, a larger mandrel is used to compact the aggregate near the top of the column.
One long-standing problem that has been sought to be solved is that in soft or unstable soil environments, a formed column cavity may tend to distort, cave-in, or become otherwise damaged as the column is formed in situ. The sidewall collapse occurs as the prior art tamper is driven downward thereby applying lateral pressure to the side of the cavity as the aggregate is compressed. This pressure results in a rotation of the soft soils in the vicinity around the tamper head and results in sidewall collapse above the elevation of the tamper head. Sidewall collapse must be removed during the construction process and can lead to a loss of pre-stressing. The problem is particularly vexing for relatively thick compacted lifts. Furthermore, this soil failure can slow the column construction process as extra soil must be removed or the cavity otherwise re-opened. It is therefore desirable to provide for an aggregate column construction technique which reduces the potential for damage to the column cavity (including sidewall collapse) during column construction. It is also desirable to provide for an aggregate column construction technique which allows for larger thicknesses of aggregate to be compacted per lift, thereby increasing efficiency of the process and limiting the amount of time the driven mandrel must be present in the cavity.